Remote tripping devices or protection signal transmission devices are used for transmitting protection or switching commands for distance protection in electrical high-voltage and medium-voltage networks and systems. Protection commands result, for example, in a circuit breaker being opened directly or indirectly and, in consequence in electrical disconnection of a part of the network or of the system. Conversely, other protection commands result in opening of a circuit breaker being prevented. Protection commands must be transmitted, for example, from one section of a high-voltage line to another. To do this, a transmitter in a remote tripping device produces analog signals in accordance with the protection commands, which analog signals are transmitted via a signal link. A receiver in another remote tripping device detects the transmitted signals and determines the corresponding number and nature of the protection commands.
The analog signals are, for example, in a frequency band between 0 and 4 kHz. They are either transmitted directly in this frequency band, or are modulated onto a carrier frequency and arc demodulated upstream of the receiver, or are transmitted via a digital channel and are reconstructed upstream of the receiver. In each case, an analog received signal is produced at the receiver, in which the presence of individual signals at a different frequency must be detected.
Depending on the nature of the protection command, different requirements are in this case placed on the signal transmission and detection, and these can be characterized by the transmission time and bandwidth, and by the following parameters:    Puc Safety and/or security value, that is to say the probability that a command is received falsely owing to disturbances on the transmission path, even though it has not actually been transmitted. A low Pmc value corresponds to high transmission safety and/or security.    Pmc Reliability value, that is to say the probability that a command which has been transmitted is not received. A low Pmc value corresponds to high transmission reliability.
Disturbances in the transmission must not simulate any commands in a quiescent situation and, on the other hand when a command occurs, must not unacceptably delay a real command, or even lead to it being lost. High safety and/or security and high reliability with a short transmission time and a narrow bandwidth at the same time are contradictory requirements. However, one variable can always be improved at the expense of the other characteristics. The compromise is governed by the application. Thus, for example, indirectly tripping protection systems require short transmission times with high reliability and reasonable safety and/or security. Applications with direct switch tripping, on the other hand, demand very high safety and/or security and reliability, with the transmission time requirements being less stringent.
FIG. 1 shows, schematically, the transmission of protection commands between remote tripping devices: a transmitter 1 has a number of command inputs 1a, 1b, 1c as inputs for binary protection commands A, B, C. On the basis of the protection commands, the transmitter 1 produces analog signals, which are transmitted via the signal link 2. A receiver 3 receives the transmitted signals, reconstructs the appropriate values of the protection commands, and emits these via command outputs 3a, 3b, 3c. The signals, which code protection commands, are also referred to generically as command signals, in contrast to a guard signal or quiescent signal, as will be explained in the following text:
FIG. 2 shows a quiescent signal and a command signal in the frequency domain and in the time domain for transmission of a single signal at a first frequency, which signal corresponds to a protection command which is to be transmitted. An amplitude axis in the illustration is annotated Amp, a frequency axis is annotated f and a time axis is annotated t. In a quiescent situation, that is to say when no protection command need be transmitted, a quiescent signal or guard signal G is transmitted continuously at a second frequency. The receiver 3 detects the presence or the absence of the command signal A and of the quiescent signal G continuously and, if the signal quality is inadequate or if both are received together or neither of the two is received, produces an alarm signal. When a command occurs, the transmitting remote tripping device interrupts the quiescent signal and transmits a command signal. In FIG. 1, this occurs between the times t1 and t2. The command signal can be transmitted at a higher level than the quiescent signal G, generally at the maximum available output power. When the receiver identifies the lack of quiescent signal G and at the same time a valid command signal with a sufficient signal quality, then the command is regarded as having been identified, that is to say that it is real.
FIG. 3 shows the transmission of a number of protection commands using one command signal per protection command. A dedicated single-tone signal at a dedicated frequency is used for each command A, B, C, D. If it is intended to transmit a number of commands at the same time, the available transmission power is shared between the corresponding single-tone signals. FIG. 3 shows the simultaneous transmission of four protection commands, in which case only a quarter of the maximum signal amplitude, as shown by the dashed line, is thus available for each of the corresponding command signals. Although the receiver characteristics relating to the transmission time, safety and/or security and reliability can be set individually for each protection command, the signal-to-noise ratio is drastically worse than when transmitting only one command signal.
FIG. 4 shows transmission by means of one, and only one, command signal per protection command A, B, and for the protection command combination A&B as well. When a command occurs, the maximum transmission power at an individual frequency is in each case available, in order to produce the maximum possible signal-to-noise ratio in the receiver. However, bandwidth and further detectors are required for each protection command C that is additionally to be transmitted, and for its possible combinations with the other protection commands A&C, B&C, A&B&C.
FIG. 5 shows transmission by means of a combination of command signal for each protection command and for each protection command combination. Combinations of the command signals are transmitted at different frequencies F1 to F5 in order to transmit one or more protection commands. By way of example, two-tone signals are transmitted, each at half the signal amplitude. An individual protection command or a specific combination of protection commands is represented or coded by each of these frequency combinations.
The methods as shown in FIGS. 4 and 5 have the advantage that they can always operate with a sufficiently high signal-to-noise ratio. However, they also have the common feature that there is no capability to take account of different safety and/or security requirements for protection commands which have been transmitted as a command combination. The presence of a command combination must, for example, always be evaluated with the safety and/or security level of that individual command which has the most stringent safety and/or security requirement. Since high safety and/or security means a longer detection time, other individual commands in the command combination, whose safety and/or security requirements are less stringent but which will be transmitted more quickly for this purpose, are unnecessarily delayed.